One very significant challenge in the design of ink-jet printers is to provide reliable paper feeding and handling. While older printers commonly use tractor-feed mechanisms, newer printers are expected to feed individual sheets of paper. This is particularly difficult with ink-jet printing since it involves the application of liquid ink directly to the paper's surface. The portions of the paper having wet ink cannot be contacted without smearing the ink. Furthermore, to apply the ink there must be an open and accessible portion of the paper which is not obstructed by paper handling components. Since it is desired to print as closely as possible to the transverse edges of the paper, and since paper widths vary, it is undesirable or impractical to grip the paper by its edges as it is being printed.
The printhead of an ink-jet printer generally comprises a plurality of nozzles aligned along the longitudinal path of the paper. The printhead is carried above the print medium path and repeatedly traverses the paper between its transverse edges. After each transverse pass of the printhead, the paper is advanced longitudinally. The area covered by a single pass of the printhead is referred to as a print zone. In order to increase printing speeds, printheads are provided with larger numbers of nozzles. This in turn increases the longitudinal dimension of the print zone associated with the printheads. As print zones become larger, however, the problems in adequately supporting paper as it passes through the print zones become greater. For example, the upper surface of the paper cannot generally be contacted as it travels through the print zone or for a short length after the print zone in which applied ink might still be wet.
Another complicating factor is the effect wet ink has on paper. The application of wet ink has a tendency to make paper bow or buckle. This effect becomes more pronounced with larger print zones, since larger areas are subject to the presence of wet ink. However, bowing to any significant degree cannot be tolerated in a high-resolution ink-jet printer. Downward bowing, away from the printhead, decreases print accuracy. Upward bowing, toward the printhead, can cause the paper to hit the printhead and to smear any applied ink. Thus, it is very desirable but also very difficult to control the precise path of paper or other print media as it passes through the print zone of an ink-jet printer.
Ink-jet printers manufactured by Hewlett-Packard Company have provided paper control through the print zone with a combination of upper and lower print media guides as described in U.S. Pat. No. 5,356,229. Paper is driven into the print zone with one or more drive rollers and associated pinch wheels. The drive rollers establish a print medium path which starts beneath the rollers, continues up around the back of the rollers, and then continues over the top of the rollers and into the print zone. The lower print media guide supports the paper from below through the print zone. An upper print media guide, positioned just upstream from the print zone, contacts the paper from above before it enters the print zone. The positioning of the components is such that the drive rollers convey the paper first in a generally downward path toward the print zone. The upper print media guide contacts the upper surface of the paper and cooperates with the drive rollers to bias the paper downwardly. Downstream from the upper print media guide, the lower print media guide contacts the paper along a transverse line beneath the print zone and biases the paper upwardly, effectively establishing a concave shape in the paper relative to the print head. The paper is desirably contacted near the print zone only by the upper and lower print media guides, and only along longitudinally-spaced lines of contact.
This configuration has been, for the most part, quite successful. However, problems do remain. Some of these problems arise because of the limited available spaces near the print zone to locate components. For instance, the upper print media guide must be very precisely located as closely as possible to the print zone. However, the printer's carriage and printhead often overhang this area. Therefore, the upper print media guide must be thinner than might otherwise be desirable for economical production with acceptable manufacturing tolerances.
Another problem is that of positioning pinch wheels over the driver rollers. Such pinch wheels would desirably be positioned to contact the drive rollers at pinch points immediately adjacent the print zone so that accurate control over the paper's longitudinal position could be achieved even when feeding the very bottom of a paper sheet through the print zone. However, this is the location occupied by the upper print media guide. Furthermore, the physical diameter of the pinch wheels limits their proximity to the print zone. As a result, the pinch points established by the pinch wheels end up being significantly upstream of the print zone, thereby reducing the capability of the printer to print on the bottom margins of paper sheets.
Control over paper edges is another problem. Even though the concave shape of the paper through the print zone reduces its tendency to bow longitudinally, applying ink at the edges of paper often causes such edges to bow upwardly. This can interfere with the printhead and cause smearing.